GNU Emacs can be programmed to emulate (more or less) most other
editors. Standard facilities can emulate these:

CRiSP/Brief (PC editor)

You can turn on keybindings to emulate the CRiSP/Brief editor with
M-x crisp-mode. Note that this rebinds M-x to exit Emacs
unless you change the user option crisp-override-meta-x. You can
also use the command M-x scroll-all-mode or set the user option
crisp-load-scroll-all to emulate CRiSP's scroll-all feature
(scrolling all windows together).

Most of the EDT emulation commands are keypad keys, and most standard
Emacs key bindings are still available. The EDT emulation rebindings
are done in the global keymap, so there is no problem switching
buffers or major modes while in EDT emulation.

"PC" bindings

The command M-x pc-bindings-mode sets up certain key bindings
for "PC compatibility"--what people are often used to on PCs--as
follows: Delete and its variants delete forward instead of
backward, C-Backspace kills backward a word (as C-Delete
normally would), M-Backspace does undo, Home and End
move to beginning and end of line, C-Home and C-End move
to beginning and end of buffer and C-Escape does
list-buffers.

PC Selection mode

The command M-x pc-selection-mode enables a global minor mode
that emulates the mark, copy, cut and paste commands of various other
systems--an interface known as CUA. It establishes the keybindings
of PC mode, and also modifies the bindings of the cursor keys and the
next, prior, home and end keys. It does not
provide the full set of CUA keybindings--the fundamental Emacs keys
C-c, C-v and C-x are not changed.

The standard keys for moving around (right, left,
up, down, home, end, prior, next,
called "move-keys") deactivate the mark in PC selection mode.
However, using Shift together with the "move keys" activates
the region over which they move. The copy, cut and paste functions
are available on C-insert, S-delete and S-insert
respectively.

The s-region package provides similar, but less complete,
facilities.

TPU (DEC VMS editor)

M-x tpu-edt-on turns on emulation of the TPU editor emulating EDT.

vi (Berkeley editor)

Viper is the newest emulator for vi. It implements several levels of
emulation; level 1 is closest to vi itself, while level 5 departs
somewhat from strict emulation to take advantage of the capabilities of
Emacs. To invoke Viper, type M-x viper-mode; it will guide you
the rest of the way and ask for the emulation level. *note Viper: (viper)Top.

vi (another emulator)

M-x vi-mode enters a major mode that replaces the previously
established major mode. All of the vi commands that, in real vi, enter
"input" mode are programmed instead to return to the previous major
mode. Thus, ordinary Emacs serves as vi's "input" mode.

Because vi emulation works through major modes, it does not work
to switch buffers during emulation. Return to normal Emacs first.

If you plan to use vi emulation much, you probably want to bind a key
to the vi-mode command.

vi (alternate emulator)

M-x vip-mode invokes another vi emulator, said to resemble real vi
more thoroughly than M-x vi-mode. "Input" mode in this emulator
is changed from ordinary Emacs so you can use ESC to go back to
emulated vi command mode. To get from emulated vi command mode back to
ordinary Emacs, type C-z.

This emulation does not work through major modes, and it is possible
to switch buffers in various ways within the emulator. It is not
so necessary to assign a key to the command vip-mode as
it is with vi-mode because terminating insert mode does
not use it.

*note VIP: (vip)Top, for full information.

WordStar (old wordprocessor)

M-x wordstar-mode provides a major mode with WordStar-like
keybindings.